Connecting rod weight order

[JW2000]

Hi, I remember reading that the heaviest rods should be installed near the flywheel and lightest sets near the pulley. However, when cap and rod are weighed separately an issue arises. when observing 2 opposing cylinders rod sets,cyl 3 and 6, 1 set can have a significantly heavier cap with a much lighter rod and the other opposing set can have a much lighter cap with heavier rod, even though total weights are still close.. Weights are as follow.

Rod + Cap = Total
set #1 438 + 154 = 592
set #2 431 + 160 = 591
set #3 438 + 152 = 590
set #4 432 + 158 = 590
set #5 424 + 165 = 589
set #6 425 + 164 = 589

if I go by total weight,
then Set#1 goes to cyl #3 since its closest to flywheel and heaviest.
set#2 goes to cyl #6, sine its second heaviest, but notice the big difference in rod and cap weight between both sets.

Other option is, if i go by heaviest rod weight, and keep rod and cap rate close to each other.
then set #1 goes to Cyl #3, being the heaviest.
set #3 goes to Cyl #6, not the second heaviest but matches rod and cap closer.


Pistons and pins are all identical weights

any recommendation? ty

[otto_kretschmer]

On the sets that are opposing each other, they have identical (or nearly) masses but different rod and cap mass, they will have different centers of gravity, and that will create an unbalance. I don't think you will notice the unbalance but if you had a sensitive enough instrument, you could measure it.

BMW boxer twin motorcycles have two pistons/rods going in opposite directions and they balance each other on the axis of the crank. But the rod journals on the crank are offset by about 2" and that creates a couple along the vertical axis. On some of the older bikes nothing was done. On the R90S, two holes were drilled in one of the counterweights and a tungsten pin was pressed in to create an unbalance to offset this couple. One the newer oil heads and wet heads, BMW resorted to counterbalance shafts.

I like the idea of putting the heaviest set nearest to the flywheel but I wouldn't lose any sleep over it. The cam train isn't balanced and there are other forces going on that we don't address. There is an oil cloud around the crank the moves around and who knows about the airflow behind the pistons.

You can see the tungsten pins in this pic just below the right main journal

[icarp]

It is best the weigh the big end and then weigh the small end
Do a search and you will see the set up for this , chain , bearing, scale , level rod ,
Then trim parts , then mate to crank
Ian

[targa72e]

Fixture for balancing rods looks like this.



Once you have all the big ends weighing the same you can balance the overall weight by removing the rest from the small end.

John